text: Townsend's early experimental apparatus consisted of planar parallel plates forming two sides of a chamber filled with a gas. A direct current high-voltage source was connected between the plates; the lower plate being the cathode while the other was the anode. He forced the cathode to emit electrons using the photoelectric effect by irradiating it with X-rays, and he found that the current  flowing through the chamber depended on the electric field between the plates. However, this current showed an exponential increase as the plate gaps became small, leading to the conclusion that the gas ions were multiplying as they moved between the plates due to the high electric field.
  Townsend observed currents varying exponentially over ten or more orders of magnitude with a constant applied voltage when the distance between the plates was varied. He also discovered that gas pressure influenced conduction: he was able to generate ions in gases at low pressure with a much lower voltage than that required to generate a spark. This observation overturned conventional thinking about the amount of current that an irradiated gas could conduct.
  The experimental data obtained from his experiments are described by the following formula
  where
  * is the current flowing in the device,
  * is the photoelectric current generated at the cathode surface,
  * is Euler's number
  * is the first Townsend ionisation coefficient, expressing the number of ion pairs generated per unit length (e.g. meter) by a negative ion (anion) moving from cathode to anode,
  * is the distance between the plates of the device.
  The almost constant voltage between the plates is equal to the breakdown voltage needed to create a self-sustaining avalanche: it decreases when the current reaches the glow discharge regime. Subsequent experiments revealed that the current  rises faster than predicted by the above formula as the distance  increases: two different effects were considered in order to better model the discharge: positive ions and cathode emission.